Electronic Grille Cipher?

[Deranged Mutant]

An idea occurred to me the other day, for hiding multiple
texts in one file using multiple keys.  The gist of it is to
take a reasonably large file of random data and then to hide
the bytes of a message in scattered locations.  The method of
determining where each byte is would be based on a good cipher
which for each iteration would return a relative offset from
the last location in the file.

It's the electronic form of those ciphers where you have a
message in a grid, the key being random holes punched in a
card and the rest of the boxes filled with junk... I think it's
called a Grille Cipher, right?

If the file is reasonably large and the messages reasonably
small, then multiple messages with multiple keys can be hidden
in the same file.  Thus one passphrase can decrypt an innocuous
message while another decrypts the real message. (Some care
should be taken to make sure there are no collisions.)

If an unencrypted plaintext was intermixed in a "truly random"
file this way, it would be difficult for an attacker to extract
it (though it's nicer to intermix an encrypted file...).

This method could also be applied to stego when hiding a file
in graphics or sound files since an attacker who suspects a
stegoed file would have trouble detecting a PGP header.

Any comments? Has this been thought of before?

--Rob